Studies On Techniques And Experiments Of The Bolts Tightening For Engine's Cylinder Head

Bolt connection is the main connection mode in the engine assembly process. Recently many companies use traditional torque control method in their assembly process of engine's cylinder head. The deviation range of the prestressing force is in±35% because of the interface friction, screw thread friction, tightening speed of bolt assembly, shape precision of screw thread and tightening tools in the bolt connection process. It would be happened that the bolt snap because of the excessive torque, although the prestressing force is suitable in the production. There also will be happened that the torque achieves the limit range when the prestressing force is still not enough. There are many bolts on the engine cylinder head, and it should make sure the seal of the cylinder by the assembly technology. It requires the bolt joint prestressing not only abundance, but also well distributed in the production. But the torque method is affected by many factors, so the deviation range of prestressing force different batches of cylinder head bolts is larger even by the same technology, it can not meet the requirement. Torque-angle method means that we load initial torque on the screw first, then rotate an angle to make the material of the screw beyond the elastic limit and make it to be the best and the largest prestressing force. This can reduce the influence effected by the bad factors such as friction, and it also can make the deviation range of prestressing force to be±15 ~ 20% and distributed behind the area of bolt's yield strength.In this article, first we introduce the theory of bolt connection, included prestressing force and residual prestressing force, torque and angle, torque coefficient, friction coefficient, discussing the reason which caused the inconsistency of the torque and prestressing force. Then the force building of bolts is modeled for analyzing the force condition of the bolts in the tightening process, relationship between the prestressing force and torque, prestressing force and angle, the load exerted in work, fatigue strength and fatigue life, laying a theoretical foundation for the design of torque-angle method. Next we introduce four methods for bolt tightening at the present time: torque method, torque-angle method, yield point method, extension method, and evaluate the advantages and disadvantages. In the forth chapter according to the fact research we design the torque-angle technology preliminarily, and do some tests for our company's YC4D and YC6J series engine cylinder head bolts. In chapter five, we analyze the test result, and modulate the initial torque and angle. Then we do the second test to further perfect the torque-angle technology. At last we make clear the direction and way of yield point method'research and application, and give a reasonable prediction for the prospect of extension method's research and application.